garboard strake

The garboard strake is the lowest continuous run of wooden planking on a wooden ship's hull, sitting directly against the keel and forming the boundary between the keel and the rest of the planked hull. Its primary job is structural: it bears enormous stress from water pressure, the weight of the hull above it, and the flexing of the keel as the ship works in a seaway. On a carvel-built hull, it is a single plank running the length of the ship from stem to sternpost. On a clinker-built hull, it is the first overlapping plank.

Because of the stresses involved, garboard strakes are typically thicker than the planks above them and are made from the most durable timber available, often oak in European shipbuilding. The garboard must be curved along its length to follow the keel's line, and it must be fitted with precision so that it sits flush against the keel's sided surface. The joint between garboard and keel is sealed with caulking (driven oakum and pitch) to keep water out. Many garboards also have a rabbet or groove worked into the keel to receive the garboard's inner edge, locking it in place.

Common problems and variants

Water penetration at the garboard-to-keel seam is the most common failure point on aged wooden ships, because the constant working of the keel and the garboard against each other opens the caulking. Similarly, rot often starts in the garboard because it is constantly wet and difficult to inspect or repair without taking the ship apart. Some ship builders have added a second garboard plank, called a sister garboard, alongside the main garboard to increase strength and redundancy, though this is less common.

The term garboard derives from the Dutch gaarboard, meaning prepared or fitted board, because the plank had to be shaped and fitted with such care. The word entered English shipbuilding vocabulary by the 17th century and remains standard in wooden ship construction and restoration today. In metal shipbuilding, the equivalent structural member is the bottom strake or keel strake, which runs along the ship's centerline on the underside of the hull.